RF antennas for transmission of electromagnetic signals are preferably designed to have a size of at least one quarter of the wavelength of the transmitted signal, since this generally allows high antenna efficiency, wide bandwidth and substantially real input impedance. However, many apparatuses do not have room for an antenna large enough to satisfy this condition. For an RF signal with a frequency of e.g. 100 MHz, one quarter of the wavelength equals 0.75 m. It is thus common to utilise antennas that are considerably smaller than one quarter of the wavelength. Such antennas are generally referred to as “electrically short” or “electrically small” antennas.
Electrically short antennas inherently exhibit low radiation resistance and low efficiency. Their efficiency may be increased by reducing resistive losses in the antenna, which, however, increases the quality factor (Q) of the antenna so that the bandwidth decreases. At a typical Q of 50, the 3-dB bandwidth of the antenna is 2% of the centre frequency.
U.S. Pat. No. 4,339,827 discloses a tuning circuit for tuning a relatively small loop antenna for receiving VHF (very high frequency) television signals, e.g. in a television set. The tuning circuit comprises a variable capacitance forming a resonance circuit with the antenna and responding to a control voltage. Further impedances may be coupled to the resonance circuit by means of electronic switches. The tuning circuit allows electronic tuning of the antenna pass-band to a selected television channel within the VHF band.
Similarly, U.S. Pat. No. 4,343,001 discloses a digitally tuned electrically small antenna, where tuning components switchable to various values are connected to a terminal of the antenna. A processor monitors the output of a phase discriminator connected to a second terminal of the antenna, commands switching of the tuning components until the antenna is in tune and stores the values reached for future reference. After some period of use, the processor may have learned to switch the antenna fast between different frequencies for transmission and/or reception of signals with a corresponding frequency.
In the receivers and the transmitter disclosed in the documents mentioned above, and in many other known RF devices with electrically short antennas, the usable channel bandwidth is limited by the width of the pass-band of the tuned antenna, since received and transmitted signals with a frequency outside the pass-band will be attenuated and phase-distorted strongly by the resonance circuit.
U.S. Pat. No. 5,402,133 discloses a synthesizer radiating system with a radiating element, which has dimensions which are small relative to the wavelength of the radiated frequency band. Energy dissipation is reduced by cycling stored energy back and forth between the radiating element and a reactive storage element. Wideband operation is achieved by actively controlling solid-state switching devices that modify the energy transfers between the radiating element and the storage element within the single cycles of the carrier signal. The synthesizer radiating system allows transmitting signals with a bandwidth exceeding that of a narrowband-tuned, electrically short antenna, however with a better efficiency than that of a comparable wideband-tuned antenna.
The disclosed synthesizer radiating system comprises relatively complex circuits for controlling the switching devices. Furthermore, the switching takes place at relatively high frequencies, which causes relatively high power losses in the electronic circuits that control the switching. Therefore, the disclosed system is less suitable for low-power transmitters wherein such losses in the control circuits could easily exceed the actual transmit power.
U.S. Pat. No. 7,239,713 discloses a hearing device with a radio device for narrow-band long-wave wireless transmission of information to another hearing device. An amplifier is connected to an antenna circuit comprising a parallel circuit of a coil and a capacitor. A feedback from the antenna circuit allows the amplifier and the antenna circuit to operate as an oscillator with a specific resonance frequency and a high Q, e.g. above 10. A further capacitor is connectable to the antenna circuit by means of a switch, whereby the resonance frequency is lowered. By switching in synchronisation with a data signal to be transmitted, the coil radiates a corresponding frequency-modulated (FM) data signal. A receiver circuit in the other hearing device adapts its frequency to the carrier frequency of the transmitted signal.
In the radio device disclosed in U.S. Pat. No. 7,239,713, the antenna circuit oscillates at its resonance frequency during transmission. In the following, a transmitter, wherein the resonance frequency of the antenna circuit determines the frequency of the transmitted signal, is referred to as a “natural-frequency transmitter”.
In a natural-frequency transmitter, objects close to the antenna may affect the resonance frequency and the Q of the antenna circuit, and may thus also affect the frequency, the phase and the amplitude of the transmitted signal. In portable devices, such as e.g. hearing devices, in which low-frequency RF signals are utilised for wireless communication, the location and the electromagnetic properties of close-by objects are typically outside the control of the system designer. In such applications, the oscillator should therefore be designed to be robust against changes in impedances coupling with the antenna. Otherwise, the receiver must be able to adapt to changes of the signal frequency, phase and amplitude, which may put undesired limitations on e.g. the achievable communication range, the achievable data rate, the usable receiver circuit types, etc. This forces the designer to make a trade-off between low performance and complex radio circuits, the latter typically increasing both cost and power consumption. In systems utilising frequency-modulated signals, distortion of the transmitted frequency are reflected in the demodulated signal on the receiver side, thus worsening the problem.
Natural-frequency transmitters are generally difficult to design for robust operation, not least because the physical layout of the transmitter is critical. Sometimes, the oscillator even fails to oscillate at all, and it is not uncommon that the development of a natural-frequency transmitter requires the manufacture of several prototypes, which typically makes implementation of such transmitters in large-scale integrated circuits costly. Furthermore, minor changes to the design of a natural-frequency transmitter may have unforeseen consequences, which often prohibits successful reuse of a transmitter design in a new apparatus design.
It is an object of the present invention to provide an RF transmitter, which allows transmission of electromagnetic signals via an electrically short antenna without the above mentioned disadvantages.
Further objects are to provide a portable apparatus and a hearing aid comprising such a transmitter.
A further object is to provide a use of such a transmitter for transmission of electromagnetic signals via an electrically short antenna without the above mentioned disadvantages.
It is further an object to provide a method for operating an RF transmitter, which method allows transmission of electromagnetic signals via an electrically short antenna without the above mentioned disadvantages.